So sorry OP, my ex did similar (but for many many months) so I totally get how you're feeling. That sick feeling that something is so so wrong but you don't know what.
I think if I were you, I'd work out a bit of a plan. First get any info you need regarding finances etc (mortgage outstanding, approx house value, pensions and savings etc) in case it is leading to divorce including seeing a solicitor if possible.
Secondly, I'd read the script to arm yourself for what to look for.
Thirdly, if he's normally fairly loving and kind and it's an abrupt and fairly recent change, I think I'd want to speak to him properly about WTF is going on to see if he cracks and tells you or to move things on.
I'd tell him his behaviour (list a few examples) is making you think he no longer wants to be in the relationship. Be wary at this point, he'll likely turn every statement around to you: "well, it sounds like YOU don't want to be in the relationship...". I'd say "This is about your behaviour, I haven't changed my behaviour, YOU have and I want an explanation...". At this point he may rely on the old fall back of mental health (every man I've ever known to have an affair has blamed the behaviour change on mental health, it's the ultimate get out card).
Assuming nothing comes of the above chat, I'd start checking phones/laptops/tablets. My ex denied, denied, denied even when I had been told and told him to his face exactly who OW was (and I could tell he was lying!). I felt like I need to crack it open with evidence so found some on his work computer (phone was super locked down and never out of his sight). If you want to know for sure if there is an OW then so a phone check before "the chat" step as he will likely lock everything down after that if he hasn't already.
It's OK if all this takes some time, you don't have to make decisions straight away but I'd be aware that living with a dementor sucks your energy and confidence away pretty quickly. It creates a brain fog and anxiety and is then hard to make decisions. it may also be prudent to check finances now or soon in case he's doing anything dodgy there (syphoning out or gifting savings to family for example).
I'd also tell some people IRL to get some support. I regret keeping it all to myself with my ex (out of loyalty to him ironically and the hope that he'd snap out of it - before I knew about OW).